The proposed project will study the process of aging in epidermal cells of rats during their normal renewal and under experimental challenge produced by a standardized wounding. In so doing, improved methods of radioautographic preparation of tissues and utilization of an automated image processing, as well as scanning and transmission electron microscopy, will be employed. The study will determine the generation time of the basal progenitor cells at different points in the lifespan of the animal and the extent to which the wound healing process is affected by aging. If certain phases of the cell cycle are affected, as has been shown by previous studies, and if the regenerative capacity is diminished as a function of aging, the question of whether or not these changes are accompanied by changes in the percentage of cells in cycle and the relative compartment size of G1 and G2 blocked cells also will be determined.